This invention relates in general to systems for controlling fluid pressure to a transmission during gear shifts. In particular, this invention relates to an improved system for controlling fluid pressure to a transmission during gear shifts by using a MEMS microvalve to more precisely control command pressure to the transmission over a range of transmission operating parameters, such as operating temperature, voltage, and pressure.
Conventional transmissions may include pressure control valves, shift valves, and flow control valves. The pressure control valves and flow control valves control the pressure and flow of hydraulic fluid in the transmission, respectively. The shift valves, on the other hand, control various components of the transmission to achieve a desired gear ratio. The shift valves may also be used for precise pressure control during gear shifts; i.e., “shift pressure.”
Precise pressure control may improve a quality of the shift and/or a speed of the shift. When the shift pressure is too high, for example, the vehicle operator may experience rough shifting. When the shift pressure is too low, the transmission clutches may be caused to overheat. The impact of such high and low shift pressure on a conventional transmission is explained in U.S. Pat. No. 8,427,806, the disclosure of which is incorporated herein by reference.
Solenoid operated valves, which are currently the industry standard for automotive transmission control, are current controlled devices. One example of such a conventional solenoid operated valve 5 is shown in FIG. 2, which is reproduced from U.S. Pat. No. 7,192,005, the disclosure of which is also incorporated herein by reference.
Thermally actuated MEMS control valves, such as MEMS microvalves, are power controlled devices. Because they are power controlled rather than current controlled like solenoid operated valves, MEMS control valves therefore require a different control scheme and method than used for solenoid operated valves. One example of MEMS control valve suitable for use with this invention is the MEMS microvalve disclosed in U.S. patent application Ser. No. 14/713,332, the disclosure of which is incorporated herein by reference.
It would be desirable to provide an improved system for controlling fluid pressure to a transmission during gear shifts by using a MEMS microvalve to more precisely control command pressure to the transmission over a range of transmission operating parameters, wherein precise control is achieved by determining and providing a required level of power to the MEMS microvalve to achieve a desired command pressure.